Effective antiviral immune responses require efficient communications between innate and ad immunity. By recognizing virus-infected cells via specific non-polymorphic receptors, natural killer (NK) cells are a central component of innate immunity; as the critical presenters of antigen to na[unreadable]ve T cells, dendritic cells (DCs) are crucial for initiating adaptive immunity. Recently, we identified a unique rare hybrid immune cell type, termed an interferon-producing killer DC (IKDC), that expresses both NK-related and DC-related molecules. Murine IKDCs represent ~0.1-0.4% of the total immune cells in lymphoid organs including spleen and lymph nodes. They are related to but distinct from conventional DCs (cDCs), plasmacytoid DCs (pDCs) and NK cells: IKDCs transiently express NK activity against the target cells using NKG2D as a recognition receptor following toll-like receptor (TLR-9) activation. This NK activity subsequently diminishes, along with a loss of NKG2D. As NK activity is lost, DC-like antigen-presenting activity is gained. This switch-of-function is also associated with an upregulation of surface MHC class II and costimulatory molecules. Our reported findings show that activated IKDCs process and present antigens in vitro and in vivo. Depending upon the activation stimuli, IKDCs produce various amounts of IFN-a, TNF-a, IL-12 and/or IFN-?. By virtue of their dual capacity to exhibit NK and antigen-presenting activities, here we postulate that the newly discovered IKDCs represent a key player in anti-viral immunity. As presented in this application, our latest preliminary data show that infection by influenza virus A/PR/8/34 (FLU) or murine cytomegalovirus (MCMV) expands IKDC population in vivo. To this end, the central focus of this proposal is to characterize the undefined role of IKDCs during infection. Specifically, we aim (1) To test the hypothesis that virus or virus-infected cells can directly activate IKDCs to mediate innate antiviral immunity. Experimentally, this will be accomplished by examining the ability of viruses (i.e. FLU, MCMV) or FLU-infected fibroblasts to directly activate IKDCs in vitro, free from in vivo effects such as potential cross-talks with other immune cell populations, cytokine/chemokine effects, etc, to secrete antiviral cytokines and exhibit natural killing activity. (2) To test the hypothesis that virus or virus-infected cells can directly activate IKDCs to mediate adaptive antiviral immunity. We will examine if coculturing IKDCs with virus (FLU or MCMV) or FLU-infected fibroblasts upregulates their expression of DC markers and induces their ability to process and present antigens to CD4+ and CD8+ T cells; (3) To test the hypothesis that IKDCs activate innate and adaptive immunity during in vivo FLU and MCMV infection. Taken collectively, not only does the proposed work promise to improve our fundamental understanding of IKDCs but may also lead to novel strategies of fine-tuning our immune system against viral infections. [unreadable] [unreadable] [unreadable]